Alkane sulfonic and disulfonic acids were first used as additives for electrolytic baths in 1930, at the Politecnico of Milan. After the Second World War American, French, German, Polish and Soviet researchers reported and claimed disulfonic acids and their salts as improvers of cathode efficiency in chromium plating baths. However, application of these types of baths on a large scale over a period of time revealed inferior properties compared to traditional baths, in that they cause accelerated corrosion of the anode (an alloy of lead).
The mechanism that leads to these drawbacks is described as follows:
Acidic dissolution of PbO.sub.2 due to the polarization of acid concentration:
PbO.sub.2 +2H.sup.+ =PbO.sup.2+ +H.sub.2 O PA1 PbO.sub.2 +H.sub.2 O.sub.2 +2H.sup.+ =Pb.sup.2+ +O.sub.2 +2H.sub.2 O (the reformation and the stabirization of the PbO.sub.2 is, on the contrary, favored by a deficit of free acid: Pb.sup.2+ +O.sub.2 +H.sub.2 O=PbO.sub.2 +H.sub.2 O.sub.2 +2H.sup.+). PA1 a) The aminoalkanesulfonic and the heterocyclic bases are added to the chromium plating baths containing Alkanedisulfonic and Alkanesulfonic acids and salts, in such concentrations as to give a Faraday output of 15-16% constant (not of interest in this patent which claims other parameters). PA1 b) The corrosion inhibitors, chemical compounds, added to the chromic solutions containing Alkanesulfonic and Alkanedisulfonic acids and salts, drastically reduce the corrosion rate of anodes immersed in them, shifting the corrosion potential to values nobler than the primary potential, or increasing the overload of the anodic or cathode process or of both simultaneously according to their chemical nature. PA1 n=integer from 1 to 12 PA1 X=NH.sub.2 PA1 and salts thereof, PA1 and nitrogen containing heterocyclic bases and/or their complexes with CrO.sub.3. PA1 n=integer from 1 to 12 PA1 Y=H or SO.sub.3 H; PA1 and salts thereof. PA1 a) the technique of E. Haring and W. Blum; PA1 b) Method of C. Pam.
Reaction of the lead oxide favored by the excess of acidity with H.sub.2 O.sub.2 formed at the anode
The anode degradation rate is further increased by the fact that the Pb.sup.2+ ions formed are removed from the equilibrium by the formation of stable complexes with ions in solution--for instance traces of halides and degradation products of the organic acids. Many proposals have been suggested to eliminate the drawbacks described above, by chemical and electrical means, but with unsatisfactory results.